(Al.sub.x Ga.sub.1- x).sub.0.5 In.sub.0.5 P (0 (0 .ltoreq. x .ltoreq. 1) crystal which is lattice-matched to a GaAs substrate is an important material for a visible light semiconductor laser. Such a semiconductor laser comprises a first cladding layer of n - (A1.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5, an active layer of undoped Ga.sub.0.5 In.sub.0.5 P (x = 0), and a second cladding layer of p - (A1.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P which provide a double heterostructure grown on a (001) plane of an n - GaAs substrate (simply defined "(001) GaAs substrate" hereinafter). The semiconductor laser further comprises a current blocking layer of n - GaAs having a stripe-shaped groove, and a cap layer of p - GaAs successively grown on the second cladding layer, a first electrode provided on the cap layer, and a second electrode provided on a bottom surface of the GaAs substrate. In the semiconductor laser, the epitaxial layers are grown on the above explained (001) GaAs substrate or a plane oriented in regard to the (001) plane by two degrees (2.degree.), for instance, as described on pages 704 to 711 of "IEEE Journal of Quantum Electronics, Vol. QE-23, No. 6, June 1987" and pages 1084 and 1085 of "Electronics Letters, Vol. 21, 1985".
In operation, a predetermined voltage is applied across the first and second electrodes of the semiconductor laser, so that current is injected limitedly into the stripe region in the presence of the current blocking layer. As a result, visible light having a wavelength which is determined dependent on a bandgap energy Eg of the active layer is emitted from the semiconductor.
In the fabrication of such a visible light semiconductor laser, metalorganic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE) is generally adopted for the crystal growth of respective layers. In a case of using MOVPE, Ga.sub.0.5 In.sub.0.5 P (x = 0) crystal lattice-matched to a GaAs substrate is grown on a (001) GaAs substrate or a two degree-oriented plane thereto under the crystal growth condition that a growth temperature ranges 550 to 750.degree. C., a V/III ratio is less than 500, and a flow amount of a raw material gas is kept constant. A bandgap energy Eg of the Ga.sub.0.5 In.sub.0.5 P crystal thus grown thereon is varied from 1.85 to 1.91 eV dependent on a combination of a growth temperature and a V/III ratio in such a state that the Ga.sub.0.5 In.sub.0.5 P crystal is lattice-matched to the GaAs substrate. The variation of the bandgap energy Eg is caused by the difference of atomic distributions on column III sublattices. That is, where a superlattice structure in which Ga and In are orderly distributed on the column III sublattices is grown, the bandgap energy Eg is 1.85 eV, and where a crystal in which Ga and In are randomly distributed on the column III sublattices is grown, the bandgap energy Eg is 1.9 eV, as described on pages 673 to 675 of "Appl. Phys. Lett. 50(11), 16 Mar. 1987". Even in a case of using MBE, the variation of a bandgap energy Eg occurs in a range of 60 meV dependent on a combination of a growth temperature and a V/III ratio, where the Ga.sub.0.5 In.sub.0.5 P layer is grown on the (001) GaAs substrate or the two degree-oriented plane. Such a phenomenon that a bandgap energy Eg varies dependent on a growth condition is also observed in a (Al.sub.x Ga.sub.1- x).sub.0.5 In.sub.0.5 P (x .noteq. 0) crystal, for instance, a Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P (x = 0.5) crystal which is grown in either MOVPE or MBE on a GaAs substrate.
For these reasons, a growth temperature and a V/III ratio are strictly controlled to be in a narrow tolerance in growing the Ga.sub.0.5 In.sub.0.5 P crystal having the maximum bandgap energy Eg of 1.9 eV, in which an average value of Ga composition is 0.5 in the crystal, by using MOVPE or MBE. Therefore, it is difficult and even more impossib1e to grow a Ga.sub.0.5 In.sub.0.5 P crysta1 layer having a room temperature bandgap energy of 1.91 eV, for instance, where a semiconductor laser including an active layer of a Ga.sub.0.5 In.sub.0.5 P crystal is fabricated because a growth condition is restricted even in a crystal growth of layers other than the Ga.sub.0.5 In.sub.0.5 P crystal, so that the growth condition tends to deviate from the aforementioned narrow tolerance. As a result, the bandgap energy Eg of the Ga.sub.0.5 In.sub.0.5 P active layer becomes less than 1.91 eV, so that a semiconductor laser having the minimum oscillation wavelength of approximately 650 nm which is inherent to the mixed crystal is not obtained, and the oscillation wavelength is increased up to the range of 670 to 690 nm due to the deviated growth condition. This results in the deterioration of stability and reproducibility in a oscillation wavelength in a semiconductor laser. The same disadvantage is induced in a semiconductor laser including an active layer of (Al.sub.x Ga.sub.1- x).sub.0.5 In.sub.0.5 P in which an oscillation wavelength shorter than that of the Ga.sub.0.5 In.sub.0.5 P active layer is obtained.